Cerebrospinal Fluid (CSF) Analysis

Cerebrospinal fluid (CSF) analysis

Definition

Cerebrospinal fluid (CSF) analysis is a set of laboratory tests that examine a sample of the fluid surrounding the brain and spinal cord. This fluid is an ultrafiltrate of plasma. Plasma is the liquid portion of blood. CSF is clear and colorless. It contains glucose, electrolytes, amino acids, and other small molecules found in plasma, but it has very little protein and few cells. CSF protects the central nervous system from injury, cushions it from the surrounding bone structure, provides it with nutrients, and removes waste products by returning them to the blood.

CSF is withdrawn from the subarachnoid space through a needle by a procedure called a lumbar puncture or spinal tap. CSF analysis includes tests in clinical chemistry, hematology, immunology, and microbiology. Usually three or four tubes are collected. The first tube is used for chemical and/or serological analysis, and the last two tubes are used for hematology and microbiology tests. This method reduces the chances of a falsely elevated white cell count caused by a traumatic tap (bleeding into the subarachnoid space at the puncture
site), and contamination of the bacterial culture by skin germs or flora.

Purpose

The purpose of a CSF analysis is to diagnose medical disorders that affect the central nervous system. Some of these conditions are as follows:

meningitis and encephalitis , which may be viral, bacterial, fungal, or parasitic infections

metastatic tumors (e.g., leukemia) and central nervous system tumors that shed cells into the CSF

Routine examination of CSF includes visual observation of color and clarity and tests for glucose, protein, lactate, lactate dehydrogenase, red blood cell count, white blood cell count with differential, syphilis serology (testing for antibodies indicative of syphilis), Gram stain, and bacterial culture. Further tests may need to be performed depending upon the results of initial tests and the presumptive diagnosis.

GROSS EXAMINATION Color and clarity are important diagnostic characteristics of CSF. Straw, pink, yellow, or amber pigments (xanthochromia) are abnormal and indicate the presence of bilirubin, hemoglobin, red blood cells, or increased protein. Turbidity (suspended particles) indicates an increased number of cells. Gross examination is an important aid to differentiating a subarachnoid hemorrhage from a traumatic tap. The latter is often associated with sequential clearing of CSF as it is collected; streaks of blood in an otherwise clear fluid; or a sample that clots.

GLUCOSE CSF glucose is normally approximately two-thirds of the fasting plasma glucose. A glucose level below 40 mg/dL is significant and occurs in bacterial and fungal meningitis and in malignancy.

PROTEIN Total protein levels in CSF are normally very low, and albumin makes up approximately two-thirds of the total. High levels are seen in many conditions, including bacterial and fungal meningitis, tumors, subarachnoid hemorrhage, and traumatic tap.

LACTATE The CSF lactate is used mainly to help differentiate bacterial and fungal meningitis, which cause increased lactate, from viral meningitis, which does not.

LACTATE DEHYDROGENASE This enzyme is elevated in bacterial and fungal meningitis, malignancy, and subarachnoid hemorrhage.

WHITE BLOOD CELL (WBC) COUNT The number of white blood cells in CSF is very low, usually necessitating a manual WBC count. An increase in WBCs may occur in many conditions, including infection (viral, bacterial, fungal, and parasitic), allergy, leukemia, hemorrhage, traumatic tap, encephalitis, and Guillain-Barré syndrome. The WBC differential helps to distinguish many of these causes. For example, viral infection is usually associated with an increase in lymphocytes, while bacterial and fungal infections are associated with an increase in polymorphonuclear leukocytes (neutrophils). The differential may also reveal eosinophils associated with allergy and ventricular shunts; macrophages with ingested bacteria (indicating meningitis), RBCs (indicating hemorrhage), or lipids (indicating possible cerebral infarction); blasts (immature cells) that indicate leukemia; and malignant cells characteristic of the tissue of origin. About 50 percent of metastatic cancers that infiltrate the central nervous system and about 10 percent of central nervous system tumors will shed cells into the CSF.

RED BLOOD CELL (RBC) COUNT While not normally found in CSF, RBCs will appear whenever bleeding has occurred. Red cells in CSF signal subarachnoid hemorrhage, stroke , or traumatic tap. Since white cells may enter the CSF in response to local infection, inflammation, or bleeding, the RBC count is used to correct the WBC count so that it reflects conditions other than hemorrhage or a traumatic tap. This is accomplished by counting RBCs and WBCs in both blood and CSF. The ratio of RBCs in CSF to blood is multiplied by the blood WBC count. This value is subtracted from the CSF WBC count to eliminate WBCs derived from hemorrhage or traumatic tap.

GRAM STAIN The Gram stain is performed on a sediment of the CSF and is positive in at least 60 percent of cases of bacterial meningitis. Culture is performed for both aerobic and anaerobic bacteria. In addition, other stains (e.g. the acid-fast stain for Mycobacterium tuberculosis, fungal culture, and rapid identification tests (tests for bacterial and fungal antigens) may be performed routinely.

SYPHILIS SEROLOGY Syphilis serology involves testing for antibodies that indicate neurosyphilis. The fluorescent treponemal antibody-absorption (FTA-ABS) test is often used and is positive in persons with active and treated syphilis. The test is used in conjunction with the VDRL test for nontreponemal antibodies, which is
positive in most persons with active syphilis, but negative in treated cases.

Description

Lumbar puncture is performed by inserting the needle between the fourth and fifth lumbar vertebrae (L4-L5). This location is used because the spinal cord stops near L2, and a needle introduced below this level will miss the cord. In rare instances, such as a spinal fluid blockage in the middle of the back, a physician may perform a spinal tap in the cervical spine.

Precautions

In some circumstances, a lumbar puncture to withdraw a small amount of CSF for analysis may lead to serious complications. Lumbar punctures should be performed only with extreme caution and only if the benefits are thought to outweigh the risks. In people who have bleeding disorders, lumbar puncture can cause hemorrhage that can compress the spinal cord. If there is increased spinal column pressure, as may occur with a brain tumor and other conditions, removal of CSF can cause the brain to herniate, compressing the brain stem and other vital structures and leading to irreversible brain damage or death. Meningitis may be caused by bacteria introduced during the puncture. For this reason, aseptic technique must be followed strictly, and a lumbar puncture should never be performed at the site of a localized skin lesion.

Specimens should be handled with caution to avoid contamination with skin flora. They should be refrigerated if analysis cannot be performed immediately.

Aftercare

After the procedure, the site of the puncture is covered with a sterile bandage. The patient should remain lying for four to six hours after the lumbar puncture. Vital signs should be monitored every 15 minutes for four hours, then every 30 minutes for another four hours. The puncture site should be observed for signs of weeping or swelling for 24 hours. The neurological status of the patient should also be evaluated for such symptoms as numbness and/or tingling in the lower extremities.

Risks

The most common side effect after the removal of CSF is a headache . This occurs in up to 40 percent of children. It is caused by a decreased CSF pressure related to a small leak of CSF through the puncture site. These headaches usually are a dull pain , although some people report a throbbing sensation. A stiff neck and nausea may accompany the headache. Lumbar puncture headaches typically begin within two days after the procedure and persist from a few days to several weeks or months.

Normal results

The normal results include the following:

gross appearance: normal CSF, clear and colorless

CSF opening pressure: in children older than six to eight years, 90–180 mm H2O; in infants and younger children, 10–100 mm H2O

specific gravity: 1.006–1.009

glucose: 40–80 mg/dL

total protein: 15–45 mg/dL

LD: 1/10 of serum level

lactate: less than 35 mg/dL

leukocytes (white blood cells): 0–6/microL (adults and children); up to 19/microL in infants; up to 30/microL (newborns)

differential: 60–80 percent lymphocytes; up to 30 percent monocytes and macrophages; other cells 2 percent or less. Monocytes and macrophages are somewhat higher in neonates, and make up as much as 80 percent or more, with only 20 percent or less being lymphocytes.

Gram stain: negative

culture: sterile

syphilis serology: negative

red blood cell count: normally, none unless the needle passes though a blood vessel on route to the CSF

Parental concerns

If the child is anxious or uncooperative, a short-acting sedative may be given. Patients receive a local anesthetic to minimize any pain in the lower back from inserting the needle.

When to call the doctor

If the child does not respond to the parents, if the puncture site continues to leak a watery fluid, or the puncture site appears red and swollen, or has other signs of infection, then the doctor should be notified.

KEY TERMS

Demyelination—Disruption or destruction of the myelin sheath, leaving a bare nerve. It results in a slowing or stopping of the impulses that travel along that nerve.

Encephalitis—Inflammation of the brain, usually caused by a virus. The inflammation may interfere with normal brain function and may cause seizures, sleepiness, confusion, personality changes, weakness in one or more parts of the body, and even coma.

Guillain-Barré syndrome—Progressive and usually reversible paralysis or weakness of multiple muscles usually starting in the lower extremities and often ascending to the muscles involved in respiration. The syndrome is due to inflammation and loss of the myelin covering of the nerve fibers, often associated with an acute infection. Also called acute idiopathic polyneuritis.

Meningitis—An infection or inflammation of the membranes that cover the brain and spinal cord. It is usually caused by bacteria or a virus.

Multiple sclerosis—A progressive, autoimmune disease of the central nervous system characterized by damage to the myelin sheath that covers nerves. The disease, which causes progressive paralysis, is marked by periods of exacerbation and remission.

Spinal canal—The opening that runs through the center of the spinal column. The spinal cord passes through the spinal canal. Also called the vertebral canal.

Subarachnoid—Referring to the space underneath the arachnoid membrane, the middle of the three membranes that sheath the spinal cord and brain.

Treponeme—A term used to refer to any member of the genus Treponema, which is an anaerobic bacteria consisting of cells, 3–8 micrometers in length, with acute, regular, or irregular spirals and no obvious protoplasmic structure.

Vertebrae—Singular, vertebra. The individual bones of the spinal column that are stacked on top of each other. There is a hole in the center of each bone, through which the spinal cord passes.

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Cerebrospinal Fluid (CSF) Analysis

Gale Encyclopedia of Medicine, 3rd ed.
COPYRIGHT 2006 Thomson Gale

Cerebrospinal Fluid (CSF) Analysis

Definition

Cerebrospinal fluid (CSF) analysis is a laboratory test to examine a sample of the fluid surrounding the brain and spinal cord. This fluid is a clear, watery liquid that protects the central nervous system from injury and cushions it from the surrounding bone structure. It contains a variety of substances, particularly glucose (sugar), protein, and white blood cells from the immune system. The fluid is withdrawn through a needle in a procedure called a lumbar puncture.

Purpose

The purpose of a CSF analysis is to diagnose medical disorders that affect the central nervous system. Some of these conditions include:

viral and bacterial infections, such as meningitis, West Nile virus, herpes virus, and encephalitis

tumors or cancers of the nervous system

syphilis, a sexually transmitted disease

bleeding (hemorrhaging) around the brain and spinal cord

multiple sclerosis, a disease that affects the myelin coating of the nerve fibers of the brain and spinal cord

Guillain-Barr syndrome, an inflammation of the nerves.

Early-onset Alzheimer's disease. The levels of two substances known as amyloid beta (1-42) and phosphorylated tau in CSF appear to be useful diagnostic markers for early-onset Alzheimer's.

CSF analysis is also used in forensic investigations to identify the presence of illicit drugs (e. g., heroin) or poisons in the bodies of murder, accidental overdose, or suicide victims.

Precautions

In some circumstances, a lumbar puncture to withdraw a small amount of CSF for analysis may lead to serious complications. Lumbar puncture should be performed only with extreme caution, and only if the benefits are thought to outweigh the risks, in certain conditions. For example, in people who have blood clotting (coagulation) or bleeding disorders, lumbar puncture can cause bleeding that can compress the spinal cord. If there is a large brain tumor or other mass, removal of CSF can cause the brain to droop down within the skull cavity (herniate), compressing the brain stem and other vital structures, and leading to irreversible brain damage or death. These problems are easily avoided by checking blood coagulation through a blood test and by doing a computed tomography scan (CT) or magnetic resonance imaging (MRI) scan before attempting the lumbar puncture. In addition, a lumbar puncture procedure should never be performed at the site of a localized skin infection on the lower back because the infection may be introduced into the CSF and may spread to the brain or spinal cord.

Description

The procedure to remove cerebrospinal fluid is called a lumbar puncture, or spinal tap, because the area of the spinal column used to obtain the sample is in the lumbar spine, or lower section of the back. In rare instances, such as a spinal fluid blockage in the middle of the back, a doctor may perform a spinal tap in the neck. The lower lumbar spine (usually between the vertebrae known as L4-5) is preferable because the spinal cord stops near L2, and a needle introduced below this level will miss the spinal cord and encounter only nerve roots, which are easily pushed aside.

A lumbar puncture takes about 30 minutes. Patients can undergo the test in a doctor's office, laboratory, or outpatient hospital setting. Sometimes it requires an inpatient hospital stay. If the patient has spinal arthritis, is extremely uncooperative, or obese, it may be necessary to introduce the spinal needle using x-ray guidance.

In order to get an accurate sample of cerebrospinal fluid, it is critical that a patient is in the proper position. The spine must be curved to allow as much space as possible between the lower vertebrae, or bones of the back, for the doctor to insert a lumbar puncture needle between the vertebrae and withdraw a small amount of fluid. The most common position is for the patient to lie on his or her side with the back at the edge of the exam table, head and chin bent down, knees drawn up to the chest, and arms clasped around the knees. (Small infants and people who are obese may need to curve their spines in a sitting position.) People should talk to their doctor if they have any questions about their position because it is important to be comfortable and to remain still during the entire procedure. In fact, the doctor will explain the procedure to the patient (or guardian) so that the patient can agree in writing to have it done (informed consent). If the patient is anxious or uncooperative, a short-acting sedative may be given.

During a lumbar puncture, the doctor drapes the back with a sterile covering that has an opening over the puncture site and cleans the skin surface with an antiseptic solution. Patients receive a local anesthetic to minimize any pain in the lower back.

The doctor inserts a hollow, thin needle in the space between two vertebrae of the lower back and slowly advances it toward the spine. A steady flow of clear cerebrospinal fluid, normally the color of water, will begin to fill the needle as soon as it enters the spinal canal. The doctor measures the cerebrospinal fluid pressure with a special instrument called a manometer and withdraws several vials of fluid for laboratory analysis. The amount of fluid collected depends on the type and number of tests needed to diagnose a particular medical disorder.

In some cases, the doctor must remove and reposition the needle. This occurs when there is not an even flow of fluid, the needle hits bone or a blood vessel, or the patient reports sharp, unusual pain.

Preparation

Patients can go about their normal activities before a lumbar puncture. Experts recommend that patients relax before the procedure to release any muscle tension, since the lumbar puncture needle must pass through muscle tissue before it reaches the spinal canal. A patient's level of relaxation before and during the procedure plays a critical role in the test's success.

Aftercare

After the procedure, the doctor covers the site of the puncture with a sterile bandage. Patients must avoid sitting or standing and remain lying down for as long as six hours after the lumbar puncture. They should also drink plenty of fluid to help prevent lumbar puncture headache, which is discussed in the next section.

Risks

For most people, the most common side effect after the removal of CSF is a headache. This occurs in 10-30% of adult patients and in up to 40% of children. It is caused by a decreased CSF pressure related to a small leak of CSF through the puncture site. These headaches usually are a dull pain, although some people report a throbbing sensation. A stiff neck and nausea may accompany the headache. Lumbar puncture headaches typically begin within two days after the procedure and persist from a few days to several weeks or months.

Since an upright position worsens the pain, patients with a lumbar puncture headache can control the pain by lying in a flat position and taking a prescription or non-prescription pain relief medication, preferably one containing caffeine. In rare cases, the puncture site leak is "patched" using the patient's own blood.

People should talk to their doctor about complications from a lumbar puncture. In most cases, this test to analyze CSF is a safe and effective procedure. Some patients experience pain, difficulty urinating, infection, or leakage of cerebrospinal fluid from the puncture site after the procedure.

Normal results

Normal CSF is clear and colorless. It may be cloudy in infections; straw- or yellow-colored if there is excess protein, as may occur with cancer or inflammation; blood-tinged if there was recent bleeding; or yellow to brown (xanthochromic) if caused by an older instance of bleeding.

A series of laboratory tests analyze the CSF for a variety of substances to rule out possible medical disorders of the central nervous system. The following are normal values for commonly tested substances:

CSF pressure: 50-180 mm H2O

glucose: 40%-85 mg/dL

protein: 15-50 mg/dL

leukocytes (white blood cells) total less than 5 per mL

lymphocytes: 60-70%

monocytes: 30-50%

neutrophils: none

Normally, there are no red blood cells in the CSF unless the needle passes though a blood vessel on route to the CSF. If this is the case, there should be more red blood cells in the first tube collected than in the last.

Abnormal results

Abnormal test result values in the pressure or any of the substances found in the cerebrospinal fluid may suggest a number of medical problems including a tumor or spinal cord obstruction; hemorrhaging or bleeding in the central nervous system; infection from bacterial, viral, or fungal microorganisms; or an inflammation of the nerves. It is important for patients to review the results of a cerebrospinal fluid analysis with their doctor and to discuss any treatment plans.

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Cerebrospinal Fluid (CSF) Analysis

Gale Encyclopedia of Surgery: A Guide for Patients and Caregivers
COPYRIGHT 2004 The Gale Group Inc.

Cerebrospinal fluid (CSF) analysis

Definition

Cerebrospinal fluid (CSF) analysis is a set of laboratory tests that examine a sample of the fluid surrounding the brain and spinal cord. This fluid is an ultrafiltrate of plasma. It is clear and colorless. It contains glucose, electrolytes, amino acids, and other small molecules found in plasma, but has very little protein and few cells. CSF protects the central nervous system from injury, cushions it from the surrounding bone structure, provides it with nutrients, and removes waste products by returning them to the blood. CSF is withdrawn from the subarachnoid space through a needle by a procedure called a lumbar puncture or spinal tap. CSF analysis includes tests in clinical chemistry, hematology, immunology, and microbiology. Usually three or four tubes are collected. The first tube is used for chemical and/or serological analysis and the last two tubes are used for hematology and microbiology tests. This reduces the chances of a falsely elevated white cell count caused by a traumatic tap (bleeding into the subarachnoid space at the puncture site), and contamination of the bacterial culture by skin germs or flora.

Purpose

The purpose of a CSF analysis is to diagnose medical disorders that affect the central nervous system. Some of these conditions are:

meningitis and encephalitis, which may be viral, bacterial, fungal, or parasitic infections

metastatic tumors (e.g., leukemia) and central nervous system tumors that shed cells into the CSF

syphilis, a sexually transmitted bacterial disease

bleeding (hemorrhaging) in the brain and spinal cord

multiple sclerosis, a degenerative nerve disease that results in the loss of the myelin coating of the nerve fibers of the brain and spinal cord

Routine examination of CSF includes visual observation of color and clarity and tests for glucose, protein, lactate, lactate dehydrogenase, red blood cell count, white blood cell count with differential, syphilis serology (testing for antibodies indicative of syphilis), Gram stain, and bacterial culture. Further tests may need to be performed depending upon the results of initial tests and the presumptive diagnosis. For example, an abnormally high total protein seen in a patient suspected of having a demyelinating disease such as multiple sclerosis dictates CSF protein electrophoresis and measurement of immunoglobulin levels and myelin basic protein.

gross examination. Color and clarity are important diagnostic characteristics of CSF. Straw, pink, yellow, or amber pigments (xanthochromia) are abnormal and indicate the presence of bilirubin, hemoglobin, red blood cells, or increased protein. Turbidity (suspended particles) indicates an increased number of cells. Gross examination is an important aid to differentiating a subarachnoid hemorrhage from a traumatic tap. The latter is often associated with sequential clearing of CSF as it is collected; streaks of blood in an otherwise clear fluid; or a sample that clots.

glucose. CSF glucose is normally approximately two-thirds of the fasting plasma glucose. A glucose level below 40 mg/dL is significant and occurs in bacterial and fungal meningitis and in malignancy.

protein. Total protein levels in CSF are normally very low, and albumin makes up approximately twothirds of the total. High levels are seen in many conditions including bacterial and fungal meningitis, multiple sclerosis, tumors, subarachnoid hemorrhage, and traumatic tap.

lactate. The CSF lactate is used mainly to help differentiate bacterial and fungal meningitis, which cause increased lactate, from viral meningitis, which does not.

lactate dehydrogenase. This enzyme is elevated in bacterial and fungal meningitis, malignancy, and subarachnoid hemorrhage.

white blood cell (wbc) count. The number of white blood cells in CSF is very low, usually necessitating a manual WBC count. An increase in WBCs may occur in many conditions including infection (viral, bacterial, fungal, and parasitic), allergy, leukemia, multiple sclerosis, hemorrhage, traumatic tap, encephalitis, and Guillain-Barré syndrome. The WBC differential helps to distinguish many of these causes. For example, viral infection is usually associated with an increase in lymphocytes, while bacterial and fungal infections are associated with an increase in polymorphonuclear leukocytes (neutrophils). The differential may also reveal eosinophils associated with allergy and ventricular shunts; macrophages with ingested bacteria (indicating meningitis), RBCs (indicating hemorrhage), or lipids (indicating possible cerebral infarction); blasts (immature cells) that indicate leukemia; and malignant cells characteristic of the tissue of origin. About 50% of metastatic cancers that infiltrate the central nervous system and about 10% of central nervous system tumors will shed cells into the CSF.

red blood cell (rbc) count. While not normally found in CSF, RBCs will appear whenever bleeding has occurred. Red cells in CSF signal subarachnoid hemorrhage, stroke, or traumatic tap. Since white cells may enter the CSF in response to local infection, inflammation, or bleeding, the RBC count is used to correct the WBC count so that it reflects conditions other than hemorrhage or a traumatic tap. This is accomplished by counting RBCs and WBCs in both blood and CSF. The ratio of RBCs in CSF to blood is multiplied by the blood WBC count. This value is subtracted from the CSF WBC count to eliminate WBCs derived from hemorrhage or traumatic tap.

gram stain. The Gram stain is performed on a sediment of the CSF and is positive in at least 60% of cases of bacterial meningitis. Culture is performed for both aerobic and anaerobic bacteria. In addition, other stains (e.g. the acid-fast stain for Mycobacterium tuberculosis, fungal culture, and rapid identification tests [tests for bacterial and fungal antigens]) may be performed routinely.

syphilis serology. This involves testing for antibodies that indicate neurosyphilis. The fluorescent treponemal antibody-absorption (FTA-ABS) test is often used and is positive in persons with active and treated syphilis. The test is used in conjunction with the VDRL test for nontreponemal antibodies, which is positive in most persons with active syphilis, but negative in treated cases.

Precautions

In some circumstances, a lumbar puncture to withdraw a small amount of CSF for analysis may lead to serious complications. Lumbar punctures should be performed only with extreme caution, and only if the benefits are thought to outweigh the risks. In people who have bleeding disorders, lumbar puncture can cause hemorrhage that can compress the spinal cord. If there is increased spinal column pressure, as may occur with a brain tumor and other conditions, removal of CSF can cause the brain to herniate, compressing the brain stem and other vital structures and leading to irreversible brain damage or death. Bacteria introduced during the puncture may cause meningitis. For this reason, aseptic technique must be followed strictly, and a lumbar puncture should never be performed at the site of a localized skin lesion.

Specimens should be handled with caution to avoid contamination with skin flora. They should be refrigerated if analysis cannot be performed immediately.

Description

Lumbar puncture is performed by inserting the needle between the fourth and fifth lumbar vertabrae (L4-L5). This location is used because the spinal cord stops near L2, and a needle introduced below this level will miss the cord. In rare instances, such as a spinal fluid blockage in the middle of the back, a physician may perform a spinal tap in the cervical spine.

Aftercare

After the procedure, the site of the puncture is covered with a sterile bandage. The patient should remain lying down for four to six hours after the lumbar puncture. Vital signs should be monitored every 15 minutes for four hours, then every 30 minutes for another four hours. The puncture site should be observed for signs of weeping or swelling for 24 hours. The neurological status of the patient should also be evaluated for such symptoms as numbness and/or tingling in the lower extremities.

Risks

The most common side effect after the removal of CSF is a headache. This occurs in 10–30% of adult patients and in up to 40% of children. It is caused by a decreased CSF pressure related to a small leak of CSF through the puncture site. These headaches usually are a dull pain, although some people report a throbbing sensation. A stiff neck and nausea may accompany the headache. Lumbar puncture headaches typically begin within two days after the procedure and persist from a few days to several weeks or months.

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cerebrospinal fluid

cerebrospinal fluid The brain floats on a liquid cushion of cerebrospinal fluid (CSF) within the rigid bony skull. The CSF is contained between layers of the meninges, the membranes that enclose the brain. It fills the subarachnoid space between the delicate arachnoid mater that lines the tough fibrous outer covering, the dura mater, and the pia mater that covers the soft substance of the brain.

Since the brain floats in CSF, the fluid acts in effect to reduce the weight of the brain from some 1000 g to about 50 g, and also protects the brain from knocks on the head. However, since the brain can move within the CS, it can be damaged on the opposite side by a sudden deceleration such as in a car accident (contra coup injury).

The subarachnoid space on the outside of the brain is in continuity with a similar space around the spinal cord and also with the series of interconnected cerebral ventricles within the brain (see figure). Each of the paired lateral ventricles, in the cerebral hemispheres, contains a leaf-like, highly vascular choroid plexus. It is from these structures that the bulk of the CSF is secreted. From the lateral ventricles CSF drains into the central third ventricle, and thence through the aqueduct in the midbrain into the fourth ventricle. Both the third and fourth ventricles contribute to the flow from their own choroidal tissue. From the fourth ventricle, the CSF exits into the subarachnoid space through several openings, and fills the ‘basal cisterns’ beneath the brain. Thence the flow of CSF is mainly up and over the whole brain surface, whilst some flows down around the spinal cord. Completing the circuit back to the bloodstream, the fluid drains via the valve-like arachnoid granulations into the sagittal sinus, the large venous channel lying centrally on the top of the brain; some is also taken up into veins around spinal nerve roots and into the lymphatics of the nose.

The secretion of CSF is an active transport process that moves fluid and solutes from the blood plasma into the ventricles, the choroid plexuses being a specialized part of the blood–brain barrier. CSF secretion involves the pumping of ions and specialized ion channels, with the energy coming from glucose and oxygen in the blood. In the adult human CSF is formed at a rate of about 0.5 ml/min; the total volume is about 200 ml, of which 30 ml is in the ventricles and the remainder in the subarachnoid space. The circulation of CSF leads to the fluid being completely replaced about every 4 hours.

CSF is a weak salt solution with similar inorganic ion concentrations to plasma, but with small and significant differences, whereas the protein content is about 100 times less than that of plasma (0.5 g/litre compared to 50–70 g/litre). Abnormalities of the CSF can be important in diagnosis of some medical conditions; the fluid can be sampled by lumbar puncture from the extension of the subarachnoid space (the lumbar sac) below the lower end of the spinal cord. CSF is normally a clear, amazingly ‘bright’ fluid, and if it is cloudy or contains a raised level of protein or traces of blood this is usually an indication of brain infection, some types of brain or spinal cord tumour, or trauma.

The pressure within the brain, the intracranial pressure (ICP), is transmitted in the CSF around the spinal cord and down into the lumbar sac. With the body horizontal, it is normally low (about 10 cm H2O); it is markedly affected by posture, and raised by straining or coughing.

Blockage in the drainage pathways for CSF is one of the causes of a raised ICP, since the CSF is actively ‘pumped’ into the ventricular system. In an adult this raised pressure can cause expansion of the ventricles, with loss of neural tissue by compression against the rigid skull. In infants, when CSF drainage pathways have failed to develop normally, the raised ICP causes the head to swell because the junctions between the skull bones are not fused, resulting in hydrocephalus — ‘water on the brain’.

A raised intracranial pressure can often be recognized by looking into the eye with an ophthalmoscope, an instrument which shines a beam of light on to the retina at the back of the eye. The beam is focused onto the ‘optic disc’, where the nerves of the eye converge to pass to the brain. Normally this appears as a clearly-defined, pale concave disc, but if the pressure in the CSF is raised, the disc may bulge forwards into the cavity of the eye. As well as by blockage of CSF circulation, raised pressure can be caused by an expanding tumour or blood clot, or by swelling of a damaged or diseased brain.

The CSF also acts as a drainage route for waste products of brain metabolism, additional to their direct excretion into the capillary blood vessels everywhere in the brain across the blood–brain barrier.

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cerebrospinal fluid

cerebrospinal fluid (CSF) The fluid, similar in composition to lymph, that bathes the central nervous system of vertebrates. It is secreted by the choroid plexus into the ventricles of the brain, filling these and other cavities in the brain and spinal cord, and is reabsorbed by veins on the brain surface. Its function is to protect the central nervous system from mechanical injury.

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cerebrospinal fluid

cerebrospinal fluid Clear fluid that cushions the brain and spinal cord, giving some protection against shock. It is found between the two innermost meninges (membranes), in the four ventricles of the brain and in the central canal of the spinal cord. A small quantity of the fluid can be withdrawn by lumbar puncture to aid diagnosis of some brain and spinal cord diseases.

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cerebrospinal fluid

cerebrospinal fluid (CSF) n. the clear watery fluid that surrounds and protects the brain and spinal cord. It is contained in the subarachnoid space and circulates in the ventricles of the brain and in the central canal of the spinal cord.

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